Unusual behaviour of lithium tetraborate Li2B4O7 at low temperatures: An x-ray diffraction study

Precision measurements of the lattice parameters as well as of the intensities of Bragg reflections in the temperature range from 80 to 400 K are performed. A peculiar series of anomalies are revealed: the value of the lattice parameter jumps and the intensity of the Bragg reflections depend on the number of thermal cycles. An incommensurate superstructure with wave vector of modulation q = delta c* (delta = 0.035 at 300 K) and broad temperature range of existence (80 K > T-com < T-inc > 400 K) is discovered in the Li2B4O7 single crystals. Observations show that a phase transition to an incommensurate phase can occur during the application of a periodically varying temperature field. It is found that a periodically varying temperature field exhibits the properties of an external agent which gives rise to an incommensurate structure. The relationship between the periodic temperature field and the incommensurate effects is discussed.